1. Field of the Invention
The present invention relates to a temperature-dependent switch, having a temperature-dependent switching mechanism, a housing receiving the switching mechanism, which housing has a lower part and an upper part, two stationary contacts provided on the upper part on its inner side, each contact being connected to an associated external terminal, and a current transfer member, arranged on and moved by the switching mechanism, which current transfer member is in contact with the two stationary contacts in temperature-dependent fashion.
2. Related Prior Art
A switch of this kind is known from DE 26 44 411 C2.
The known switch has a housing with a cup-like lower part into which a temperature-dependent switching mechanism is placed. The lower part is closed off by an upper part which is held on the lower part by the latter's elevated rim. The lower part can be made of metal or of insulating material, while the upper part here is always made of insulating material.
Two rivets, whose inner heads act as stationary contacts for the switching mechanism, sit in the upper part. The switching mechanism carries a current transfer member in the form of a contact plate, which depending on the temperature is brought into contact with the two stationary contacts and then connects them electrically to one another.
The outer heads of the two rivets serve as solder terminals for leads.
The temperature-dependent switching mechanism has a bimetallic snap disk as well as a spring disk, which are penetrated centrally by a stem which carries the contact plate. The spring disk is guided peripherally in the housing, while the bimetallic snap disk is braced, depending on temperature, against the bottom of the lower part or on the rim of the spring disk, and thereby either allows the contact plate to rest against the two stationary contacts or lifts the contact plate away from the stationary contacts so that the electrical connection between the external terminals is interrupted.
This temperature-dependent switch is used in known fashion to protect electrical devices from overheating. For this purpose, the switch is connected electrically in series with the device to be protected, and is arranged mechanically on the device so that it is in thermal contact with it. Below the response temperature of the bimetallic snap disk, the contact plate rests against the two stationary contacts so that the circuit is closed and the device being protected receives power. If the temperature rises above a permissible value, the bimetallic snap disk lifts the contact plate away from the stationary contacts, thereby opening the switch and interrupting the supply of power to the device being protected so that the latter can cool off again, whereupon the switch automatically closes again.
Because of the dimensioning of the contact plate, the known switch is capable of carrying very much greater operating currents as compared with other temperature-dependent switches in which the operating current of the device being protected flows directly through the bimetallic snap disk or a spring disk associated with it, so that the known switch can be used to protect larger electrical devices with greater power consumption. Although the known switch meets many technical requirements, it nevertheless still exhibits disadvantages in certain applications.
One such disadvantage lies in the fact that after cooling, it automatically switches on again. While a switching behavior of this kind may be entirely suitable for protecting, for example, a hair dryer, such is not the case with many applications, specifically those in which the device being protected must not automatically switch back on in order to prevent damage. This applies, for example, to electric motors which are used as drive accessories.
In this connection, it is already known from DE 37 01 240 to equip a switch, having a contact bridge for connecting the two fixed contacts, with a self-hold function. In this switch a resistor is connected electrically in parallel with the two fixed contacts and carries a residual current when the switch is open, thereby heating up sufficiently such that it holds the bimetallic snap disk above its switching temperature. A disadvantage with the known switch, however, is the fact that the resistor and the bimetallic snap disk are housed in different chambers of an insulating housing, so that a separate metal bottom must be provided for heat transfer from the resistor to the bimetallic snap disk. A further disadvantage lies in the fact that the contact bridge is pressed against the fixed contacts by a helical compression spring, whose force must be continuously overcome by the bimetallic snap disk in the open state. The result of this heavy load on the bimetallic snap disk in the open state is that its switching temperature shifts unpredictably, so that both the response behavior and the self-hold function are unreliable and not reproducible.
The known switch can moreover also be equipped with a current-dependent switching function, for which purpose a further resistor is provided which is connected permanently in series with the external terminals. The operating current of the device being protected thus flows continuously through this heating resistor, which can be dimensioned so that when a specific operating current is exceeded, it causes the bimetallic snap disk to be heated to a level above its response temperature, so that in the event of an elevated operating current the switch opens even before the device being protected has heated up impermissibly. This function is also not implemented reliably in the known switch, however, since the heating resistor is arranged at an even greater physical distance from the bimetallic snap disk than the resistor for the self-hold function.
A further disadvantage of the known switch may be seen in its complex design configuration: specifically, the bimetallic snap disk actuates a switching pin which projects through the resistor for the self-hold function into a second chamber of the housing, where the contact bridge is attached to the switching pin. On the side remote from the bimetallic snap disk, there sits on the switching pin a helical compression spring which is braced at the other end internally against the housing. Fixed contacts, against which the contact bridge is pressed by the helical compression spring, project laterally into this second chamber of the known switch. The contacts in turn are pressed by projections of the plastic housing onto the resistor for the self-hold function.
With the known switch it is not apparent how production tolerances can be compensated for; assembly also appears to be extremely complex, and is probably feasible only by hand.
In addition to these "mechanical" disadvantages, however, the greatest disadvantage of the known switch lies in the poor thermal coupling between the resistors and the bimetallic snap disk.
Much better coupling between a resistor and a bimetallic snap disk is known from DE 37 10 672 C2, albeit for a switching mechanism wherein the operating current is directed through a spring disk associated with the bimetallic snap disk, meaning that the current may have much lower values than in the case of the generic switch. A two-part housing is present also in the case of the switch known from DE 37 10 672 C2, although in this case only one fixed countercontact is provided on the inner side of the cover, coacting with a movable contact that is carried by the spring disk which presses this contact against the fixed countercontact. Arranged above the spring disk is a bimetallic snap disk which, when its response temperature is exceeded, lifts the movable contact away from the fixed countercontact. In the closed state, operating current flows from the fixed countercontact into the movable contact, and from there through the spring disk into the electrically conductive lower part of the switch. In this known switch, there is arranged on the inner side of the cover a film resistor which is connected at one end to the fixed countercontact and at the other end to the electrically conductive lower part of the housing, so that when the switch is open the resistor carries a residual current, heats up as a result thereof, and holds the bimetallic snap disk at a high enough temperature so that the switch does not close again.